激光定向能量沉积Ti-6Al合金的组织与性能研究

Microstructure and Mechanical Properties of Ti-6Al Alloy Fabricated via Laser-Directed Energy Deposition

激光定向能量沉积成形钛合金具有高效快速、材料利用率高等优势,但其较大的冷却速率及温度梯度等固有特性使得成形的钛合金容易出现各向异性,因此有必要对激光定向能量沉积钛合金不同平面及不同方向上的组织与性能进行研究。选定Ti-6Al合金作为研究对象,采用光学显微镜、X射线衍射仪、扫描电镜和电子背散射衍射对激光定向能量沉积Ti-6Al合金物相、微观组织形貌和晶粒特征进行研究。结果表明,合金中只存在单一的α相,该物相为HCP结构;Ti-6Al合金不同成形高度表现出不同的微观组织特征,即沿着构建方向,底部为细小的等轴晶,中间为柱状晶,顶部为等轴晶/近等轴晶。在XOY面的平均晶粒尺寸最小,为22.2519μm;YOZ面存在最大的晶粒尺寸,为38.9408μm。在3个不同平面中,XOY面的小角度晶界体积分数最高,为19.5%;YOZ面小角度晶界体积分数最低,为12.2%。沿Z方向上,即与建造方向平行时抗拉强度最高,为669.94 MPa,相应的伸长率为13.9%;沿X方向上抗拉强度为560.55 MPa,其相应的伸长率最低,为11.4%。

Laser-directed energy deposition(LDED)of Ti alloys has many advantages,such as high efficiency and high material utilisation,but its inherent characteristics,such as a large cooling rate and temperature gradient,make LDED Ti alloys prone to anisotropy.Therefore,it is necessary to study the microstructure in different planes and the mechanical properties in different directions of LDED Ti alloys.In this paper,a Ti-6Al alloy was selected as the object of investigation.The phases,microstructures,and grain characteristics of the LDED Ti-6Al alloys were characterised via optical microscopye(OM),X-ray diffraction(XRD),scanning electron microscopye(SEM)and electron backscattering diffraction(EBSD).The results show that there is only a singleα-phase(HCP structure)in the alloy.The Ti-6Al alloy exhibits different microstructural features at different heights,i.e.,along the building direction,the bottom is composed of fine equiaxed grains,the middle is composed of columnar grains,and the top is composed of equiaxed/subequiaxed grains.The smallest average grain size of 22.2519μm can be observed in the XOY plane;the largest grain size of 38.9408μm is present in the YOZ plane.Among the three different planes,the XOY plane has the highest low-angle grain boundaries(LAGBs)volume fraction of 19.5%,whereas the YOZ plane has the lowest LAGBs volume fraction of 12.2%.The highest tensile strength of 669.94 MPa,with a corresponding elongation of 13.9%,is observed along the Z direction,i.e.,parallel to the building direction.In the X direction,the elongation is the lowest(11.4%),and the corresponding tensile strength is 560.55 MPa.